In this application, the term "document" is used to refer to any valuable sheet of paper or other body having a magnetic layer such as ink containing a magnetic pigment having specified magnetic properties.
A magnetoresistive sensing element responds to a signal recorded on a magnetic media through variation of resistance of the magnetoresistive element which intercepts the signal flux emanating from the medium. The theoretical variation in resistance for a single domain film is a parabolic function of the magnetic field strength applied to the element. Referring to FIG. 1, the ordinate of curve 10 represents the fractional change of the resistance of a typical single domain NiFe permalloy thin film magnetoresistive element as a function of magnetic field strength, H. Because of demagnetizing fields in the layer, the curve 10 deviates in the regions 12 from the "simple" parabolic response represented by dotted segments 14.
Typically, the operating point 17 of the magnetoresistive element is set to provide maximum output signal and minimum second harmonic distortion. This point occurs at the point of inflection 16 of the curve 10 in a uniform signal field. In practice, several factors cause the definition of the optimum point to be more complex, and it must generally be determined either theoretically or experimentally in a given application. A constant bias field Hb 18 applied to the magnetoresistive element establishes the operating point 17, and a varying input signal field Hs 20 applied about the operating point 17 results in an output resistance variation 22 which replicates the input signal Hs 20. The corresponding voltage drop resulting from a sense current 23 flowing through the MR element provides a voltage output.
Generally, the bias field can be produced by one of several techniques, such as by a permanent magnet, by a shunt bias, by a soft adjacent layer, by a barber pole MR configuration, or the like. Permanent magnet bias is advantageous in not requiring the use of circuitry and components external to the MR assembly to effect a large bias. Shunt bias is disclosed in the following U.S. Patents: U.S. Pat. No. 4,523,243, issued Jun. 11, 1985, inventor Billington; U.S. Pat. No. 5,159,511, issued Oct. 27, 1992, inventor Das.
A need exists for a magnetic assembly which is capable of sensing the saturation magnetic field in a magnetic layer of a document simultaneously with its production. Because the magnetic quality of magnetic ink pigment is likely to be poor, with low coercivity and remanent magnetization, it is desirable to have the ability to make in-field measurements of the documents, i.e., to read their characteristic magnetic signal with an MR sensor element while the document is being simultaneously magnetized with an external magnetic field sufficiently large to partially or fully saturate the magnetic pigment in the ink of the document. It is also desirable that such a magnetic assembly reduce the reverse magnetic field seen by a document either before or after passing the MR sensor element.
U.S. Pat. No. 3,796,859, issued Mar. 12, 1974, inventor Thompson, discloses a magnetic ink recording system including a U-shaped permanent magnet recording head for magnetizing magnetic ink on a document. A magnetoresistive (MR) sensing element is located within the yoke of the permanent magnet. The MR sensing element both is biased by the permanent magnet and senses the magnetized ink simultaneously with its production. The system disclosed in this patent is disadvantageous because the yoke ends of the permanent magnet are located close to the magnetized ink of a document and tend to demagnetize the ink before it can be sensed by a magnetic read assembly downstream from the magnetic head. The U-shaped permanent magnet can also be expensive and difficult to configure for a given application.
In U.S. patent application Ser. No. 08/114,720, filed Aug. 31, 1993, now U.S. Pat. No. 5,410,458, issued May 23, 1995, a magnetic assembly is provided for detecting and authenticating magnetic documents which provides sufficient magnetic field to partially and/or fully saturate the magnetic layer (ink) of a document which is simultaneously magnetically sensed for purposes of identification or authentication. The magnetic assembly is simple, has high intrinsic sensitivity, has a signal output with high signal-to-noise ratio, has a signal output which is independent of the speed of a magnetic document passing the assembly, and has minimal power dissipation and heating. The magnetic assembly for both magnetizing and detecting magnetic documents comprising a permanent magnet; first and second magnetically soft, high permeability pole pieces shaped to form a tapered variable gap-size magnetic circuit with said permanent magnet, wherein said permanent magnet is located at a first gap between said first and second pole pieces; a magnetoresistive (MR) sensing element which is located at or near the center of a second gap between said first and second pole pieces, wherein said second gap and said MR sensing element are proximate to a magnetic document to be sensed by said MR sensing element; and wherein said magnetic field at said MR sensing element maintains said MR element in a proper magnetic bias state for achieving adequate sensitivity for detecting magnetic fields emanating from a magnetic document to be sensed.
Absolute signal level is an important parameter in detecting counterfeit documents. Even though very sensitive magnetoresistive detectors are used, the magnetic pigment concentration in many documents is low and very high gain amplifiers must be used in the signal channel. A problem arises because sensors and amplifiers are vulnerable to sensitivity and gain drift with time and temperature changes. U.S. Pat. No. 5,301,080, issued Apr. 5, 1994, inventors Ottesen et al disclose a bias servo loop for a magneto-resistive recording head which detects distortion in the read signal from the MR head; and adjusts the operating point for the head to minimize distortion. Bias of the MR head is provided solely by a shunt bias technique which does not use permanent magnet bias.